1. Field of the Invention
This invention relates to a process for producing an O-acetyl compound such as acetic anhydride by reacting an O-methyl compound as a starting material, such as methyl acetate, with carbon monoxide by a catalytic reaction in which rhodium is used as a principal catalyst.
2. Description of the Prior Art
Acetic anhydride is used in quantity as a material for the production of cellulose acetate and, in addition, it is useful as a material for medicines, perfumes, dyes, etc.
Acetic anhydride has heretofore been produced industrially by a process in which ketene obtained by the thermal decomposition of acetic acid is reacted with acetic acid.
On the other hand, studies are being made actively to produce acetic anhydride by a reaction of carbon monoxide with methyl acetate or dimethyl ether. Although this reaction can proceed under a milder condition in a process in which rhodium is used as a principal catalyst than in processes in which other transition metal catalysts are used, it is not yet satisfactory in respect of its reaction rate in order to be used actually in the industry. Therefore, an improvement, in which a variety of reaction accelerators are incorporated in rhodium catalyst systems, has been made.
The additives for the rhodium-iodine compound (typical example being methyl iodide) catalyst system are typified by nonmetallic compounds, such as organophosphorus and organonitrogen compounds and, in addition, metallic accelerators are used in conjunction with, or separately from the nonmetallic compounds. A typical example of the metallic accelerator is chromium hexacarbonyl (Japanese Patent Laid-Open No. 115403/1976), and it is known from Japanese Patent Laid-Open Nos. 47922/1975, 52017/1975, etc., that chromium is effective. In addition, aluminum accelerators such as aluminum chloride and aluminum isopropoxide are also known (Japanese Patent Laid-Open Nos. 47922/1975 and 142234/1981). Further, zirconium (Japanese Patent Laid-Open No. 57733/1981) and titanium (Japanese Patent Laid-Open No. 142234/1981) are also known as metallic accelerators. It has been found that when an acetyl compound is produced by using these metallic accelerators, such as aluminum and chromium, the carboxylic acid salts of these metals sometimes form precipitates insoluble in the O-acetyl compound. This will bring about a considerable decrease in the concentration of a metallic accelerator in the reaction system and adversely affect the result of the carbonylation reaction. Further, a change in the composition due to precipitation is disadvantageous in repeating recirculation of a catalyst solution which is essential to the industrial production.
The solubility of an aluminum compound can be increased by proper selection of a solvent, for example, by using acetic acid as solvent, but in this case the aluminum compound is inevitably precipitated when the reaction solution is flash-distilled into a volatile component and a rhodium catalyst solution, though the decrease in the concentration in the reaction system can be prevented.
Japanese Patent Laid-Open Nos. 28980/1980, 57733/1981, etc. may be thought to be inventions in which the use of an insoluble metallic accelerator such as chromium is avoided.